This invention relates to a device for diagnosing and treating hearing disorders. More particularly, the invention relates to a device for delivering auditory sensations to the profoundly deaf and others. The device is particularly suitable for supersonic bone conduction hearing devices, diagnosis and treatment of tinnitus, diagnosis and treatment of vestibular function conditions, echo location, and determination of individual sensitivity to ultrasonic signals. The ultrasonic frequency range is about 20 kHz to about 108 kHz or higher.
Early prior art starts with the use of significantly large and bulky accelerometer devices. A next generation of devices were bimorphs from Blatec, as illustrated in FIG. 3. These devices were higher frequency acoustic generators/sensors reportedly used to sense the presence or absence of materials on an assembly line. The devices consisted of a thin piece of piezoelectric ceramic, typically 0.040 inches thick, bonded directly onto a thin sheet of aluminum, typically 0.020 inches thick. When a voltage is placed across the electrodes of the ceramic, the material either shrinks or expands in the direction of the electric field, depending on the polarity of the device. This movement of the ceramic has no beneficial output with regard to the hearing assist devices. But in response to the same electric field, the ceramic also expands or shrinks in the lateral direction, perpendicular to the electric field. However, since the physical size of the ceramic is constrained by virtue of its lamination to the aluminum sheet, the ceramic will bow the lamination into an either concave or convex form, depending on the polarity. Application of an alternating voltage will then generate vibrations at the frequency of the input signal.
The devices of FIG. 3 did not have a strong natural resonance in the 20 to 40 kHz region as required for supersonic hearing devices, nor did they have the required band width. Further, under the drive conditions required for supersonic hearing devices, these devices very rapidly either delaminated, broke the ceramic, broke the electrical connections to the ceramic electrodes, or heated up and depolarized rendering the ceramic inert.
Another generation of devices was available from Motorola, based on their development and manufacturing of piezo tweeters, as illustrated in FIG. 4. The devices were redesigned to place a strong natural resonance in the supersonic frequency range of interest, but they still lacked the desired band width. The basic concept of a bimorph, however, was the same, with exactly the same consequences.
Yet a further generation of devices was developed by ECHO Ultrasound. ECHO felt constrained to continue the development of Motorola, and did succeed in opening up the band width. Yet, the basic concept of a bimorph failed. These devices generated excessive amounts of heat (severe burn potential) and failed rapidly, typically within seconds of operation.
As a result of naval sonar during and after World War II, power ultrasonics began to be developed. More relevant to the subject at hand was the field of piezoceramic longitudinal vibrators, as shown in FIG. 5, from the book of Leon W. Camp, "Underwater Acoustics", Wiley-Interscience, 1970. Indeed, Camp writes in his book:
"The physical structure of these devices may be quite simple, consisting of a center section of active material which works between an inertial mass and a radiating diaphragm. FIG. 6.26 [FIG. 5 hereof] shows a diagrammatic arrangement of the components. In addition to the parts shown, there is usually a rod under tension through the center attached to front and back components for the purpose of holding the system under compression at all vibration levels."
An even earlier work, F. Rosenthal, and V. D. Mikuteit, (1959), IRE National Convention Record 7, Part 6, 252, and subsequently published as "Vibrations of Ferroelectric Transducer Elements Loaded by Masses and Acoustic Radiation" in IRE Transactions on Ultrasonics Engineering, February 1960, pp. 12-15 describes a mass loaded composite transducer featuring a ceramic tube and a bolt for compressive bias, as seen in FIG. 6. Rosenthal and Mikuteit go further to predict the operational resonant frequency of the device, as a function of the masses, the physical size of the ceramic tube, and the elastic constant of the ceramic. Of note, the resonant frequency is not dependent on the length of the device, as a function of resonant wavelength. The author's expression for the resonant frequency is simplified for the case of large masses. These same results are also published in W. P. Mason, "Physical Acoustics, Principles and Methods", Vol. 1, Part A, Academic Press, 1964.
In a companion work by R. S. Woollett, IRE International Convention Record 10, Part 6, p. 90, 1962, the case of air backing and fluid loading is addressed. This situation is the operational environment of the inventive device, where the fluid medium is the human body.
An alternative form of the above with a ceramic stack as compared to a ceramic tube is frequently used and also heavily described in the literature. A more recent paper by A. C. Tims, D. L. Carson, and G. W. Benthien, "Piezoelectric Ceramic Reproducibility (for 33 Mode Transducer Application), Proceedings of the 6th IEEE International Symposium on Applications of Ferroelectrics, Lehigh U., Bethlehem, Pa., pp. 6245-627, Jun. 8-11, 1986, clearly depicts the use of a stack as compared to the tube, as seen in FIG. 7.
The concept of an active component between a radiating mass and an inertial mass is still of significant interest in the transducer community, as manifested by the recent work of J. Lan, M. J. Simoneau, and S. G. Boucher, "Development of an Efficient Transducer Design Tool: Complete Finite Element Modeling of Transducer Performance Parameters on a PC", SPIE Vol. 1733, 1992, pp. 57-71. As seen in FIG. 8, they partition the components into incrementally small segments, individually and iteratively note the displacements to each segment, and predict overall device performance.
Although not easily observable from the literature, naval sonar has been utilizing these concepts for many years.
On the subject of tuning devices, virtually every textbook on transducers includes a section on tuning to impedance match or to broaden the bandwidth. No effort is made herewith to document the totality of electrical matching circuits.